1. Field of the Invention
The invention relates generally to the separating of small droplets from a continuous phase of an emulsion fluid with a coalescing element.
2. Background Art
Filtering a fluid comprising two non-miscible phases is a common operation. Typically the fluid may be stored in a container during a given time: a first phase and a second phase may separate upon gravity. The phase having the smallest density is then collected at a surface of the fluid.
However, the fluid may be under a form of an emulsion comprising a continuous phase and a dispersed phase. The emulsion may be relatively stable, in particular if a surfactant is added at a forming of the emulsion.
A separator packing allows to separate a dispersed oily phase from a continuous phase. The separator packing comprises a plurality of plates that are made of an oleophilic material so that drops of the dispersed oily phase adhere to the plates. The plates are oriented in a diagonal direction in such a way that the coalesced drops move upward along the plates upon a pressure generated by a flow of the continuous phase. The continuous phase flows through the separator packing. In case of a further phase that is solid, particles of the solid phase are intercepted by the separator packing and drop at the bottom of the separator under the effect of gravity. The separator packing hence guides the dispersed oily phase upward, the solid phase downward and let the continuous phase flow.
However, if the dispersed phase is in a form of relatively small droplets, the effect of the gravity is minimized. It may hence take a relatively long time to separate an emulsion by storing it in a container. Under such circumstances, a separator packing provides a relatively low efficiency in separating small droplets from a continuous phase.
British Patent 1 418 806 to Continental oil Co, published Dec. 24, 1975, discloses a process for separating the phases of an emulsion comprised of water and of a petroleum hydrocarbon. The process comprises passing the emulsion through a bed of polyurethane foam. Upon saturation of the polyurethane foam, the hydrocarbon dispersed phase is coalesced into droplets. If the droplets are less dense than water, the droplets rise to form an hydrocarbon layer that is above a water layer. Portions of the hydrocarbon layer and of the water layer are continually drawn off, thus effecting the separating.
U.S. Pat. No. 5,239,040 to E.R.T. Environmental Research Technology K.S.P.W. Inc. (CA), published Aug. 24, 1993, describes a polyurethane particulate liquid absorbent. The absorbent is prepared from specific reactants using a particular process. The absorbent is suitable for use in cleaning up liquids contaminated with oil droplets. The absorbent allows to intercept the oil droplets. The absorbent and the intercepted oil droplets may be separated by a centrifugation step, thus providing recovered oil, and oil-free absorbent for re-use.
International application WO02/20115 to EARTH CANADA CORP (CA), published Mar. 14, 2002, discloses a system in which a plurality of Reusable Polymer Absorbent (RPA) beds are exposed to a flow of an emulsion of oil droplets into water. The RPA beds are made of a polyurethane foam shredded in small flakes so as to increase a surface area offered to the flow. The polyurethane particulate liquid absorbent described in the U.S. Pat. No. 5,239,040 may be used.
FIG. 1A, FIG. 1B and FIG. 1C illustrate a coalescing of the oil droplets at a RPA bed according to prior art. A flow of an emulsion comprising a water continuous phase 13 and oil droplets 12 passes through RPA elements 11. As represented in FIG. 1A, the RPA elements 11 intercept oil droplets. Despite the intercepted droplets 14 that adhere to the RPA elements 11, the emulsion may still comprise free droplets 17 at an outlet of the RPA elements 11.
The intercepted droplets 14 may, upon the flow of the emulsion, form a layer 16 at a surface of the RPA elements 11, as represented in FIG. 1B.
As the layer 16 increases, the flow of the emulsion creates an increasing shearing force onto the layer 16. Consequently, large oil drops 15 may form from the layer 16 and be entrained by the flow as represented in FIG. 1C.
A creaming of an emulsion, i.e. a velocity of a given drop upon gravity is governed by Stoke's law: the large oil drops 16 move upward faster than the oil droplets (12, 17).
FIG. 2 illustrates a system for separating oil droplets from a continuous water phase according to prior art. A plurality of RPA beds (27a, 27b) are provided within a vessel 28 through which an emulsion fluid comprising oil droplets (22a, 22b, 22c) among a continuous water phase 23 may flow.
A first RPA bed 27a allows to form large oil drops 25a from the oil droplets 22a. An number of non retained oil droplets 22b may pass trough the first RPA bed 27a without adhering to any RPA element (not represented on FIG. 2) of the first RPA bed 27a. The large oil drops 25a move upward upon gravity faster than the oil droplets 22b. The large oil drops form at a surface of the water phase an oil layer 24. A recovery outlet, e.g. a recovery pipe 29 at the surface allows to recover the oil layer 24.
A similar process is iterated via a second RPA bed 27b and further RPA beds (not represented on FIG. 2) for further cleaning if necessary. It is desirable to avoid the large oil drops 25a generated at the first RPA bed 27a to enter the second RPA bed 27b. The recovering of the large oil drops 25a depends on a plurality of parameters: a velocity of the flow of emulsion fluid, a density of oil, a density of water, a distance between the first RPA bed 27a and the second RPA be 27b, and a height between the large oil drop at the first RPA bed 25a and the oil layer 24. In order to insure an efficient separating of the oil droplets 22a from the continuous water phase 23, the RPA beds (27a, 27b) may be positioned at a relatively high distance.